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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Closing an open system: Pore pressure changes in permeable edifice rock at high strain rates
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Closing an open system: Pore pressure changes in permeable edifice rock at high strain rates

机译：关闭开放系统：高应变率下可渗透建筑物岩石中的孔隙压力变化

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A permeable or open system will react as a closed system if the rocks implicated are deformed on a timescale that precludes fluid movement. Closed system ("undrained") deformation therefore leads to a failure mode dependent change in pore pressure: microcracking (dilatant behaviour) and cataclastic pore collapse (compactant behaviour) will decrease and increase pore pressure, respectively. In the dilatant regime (i.e., in the shallow edifice, <1 km depth), a decrease in pore pressure will serve to strengthen rock a process termed dilatancy hardening. However, it is shown here, using undrained triaxial deformation experiments, that the high initial porosity and microcrack density of typical edifice-forming andesites prevent dilatancy hardening. This allows the rock proximal to the magma-filled conduit in the shallow edifice to remain weak during periods of unrest when high magma strain rates could be transferred to the adjacent country rock. Although the propensity for fracturing will likely reduce the structural integrity of the edifice, fracturing of the shallow edifice may improve the outgassing efficiency of the nearby magma-filled conduit The increase in pore pressure during undrained deformation in the compactant regime (i.e., in the deep edifice, >1 km depth) could lead to pore pressure embrittlement and fracturing. Indeed, the experiments of this study show that the pore pressure increases during progressive compaction in a closed system. However, the pore pressure is prevented from reaching the critical value required to promote a dilatant response (i.e., fracturing) for two reasons. First, the rate of compaction (i.e., porosity decrease) slows as the sample is deformed at a constant strain rate, a consequence of the decay in effective pressure. Second, the emergence of microcracking as the rock approaches the compactant-dilatant transition acts as a negative feedback and prevents the rock from transiting into the dilatant field. At this point, local porosity increases due to dilatant microcracking and local porosity decreases due to cataclastic pore collapse are balanced and the rock deforms without further changes to porosity or pore pressure. This will prevent potentially destabilising brittle failure deeper in the edifice during the high strain rates that may accompany unrest and, although it precludes the formation of efficient outgassing pathways in the form of fractures, undrained deformation in the compactant regime will prevent a reduction in porosity and permeability and may therefore facilitate lateral outgassing of the conduit into the country rock. We assess the conditions (strain rate and permeability) required for drained or undrained deformation by defining a dimensionless Darcy number. Closed system or undrained deformation is likely commonplace within a volcano (strain rates in the rock adjacent to an active volcanic system can be high and textural heterogeneities can serve as barriers to fluid flow) and therefore forms an important component for a complete understanding of the mechanical response of an edifice to the stress perturbations accompanying unrest. (C) 2016 Elsevier B.V. All rights reserved.
机译：如果涉及的岩石在阻止流体运动的时间尺度上变形，则渗透性或开放性系统将作为封闭系统做出反应。因此，闭合系统（“不排水”）的变形会导致失效模式随孔隙压力的变化而变化：微裂纹（扩张性）和碎裂孔隙塌陷（致密性）将分别降低和增加孔隙压力。在膨胀状态下（即在浅层建筑物中，<1 km的深度），孔隙压力的下降将有助于岩石强化膨胀的过程。但是，这里显示了使用不排水的三轴变形实验，典型的造桥安山岩的高初始孔隙度和微裂纹密度阻止了膨胀性硬化。这样，在动荡时期，当高岩浆应变率可以转移到邻近的乡村岩石时，浅层建筑物中靠近岩浆充填管道的岩石将保持较弱。尽管破裂的倾向可能会降低建筑物的结构完整性，但浅建筑物的破裂可能会提高附近岩浆充填管道的除气效率。在压实状态下（即，在深部）不排水形变期间孔隙压力的增加建筑物，> 1 km深度）可能导致孔隙压力脆化和破裂。确实，这项研究的实验表明，在封闭系统中，渐进压实过程中孔隙压力会增加。然而，由于两个原因，阻止了孔隙压力达到促进膨胀响应（即，破裂）所需的临界值。首先，当样品以恒定的应变速率变形时，压实速率（即，孔隙率降低）变慢，这是有效压力下降的结果。第二，随着岩石接近压实膨胀曲线，微裂纹的出现起到了负反馈的作用，并阻止了岩石过渡到膨胀场中。在这一点上，由于膨胀微裂纹而引起的局部孔隙率增加，并且由于碎裂性孔隙塌陷而引起的局部孔隙率下降得到了平衡，岩石变形而孔隙率或孔隙压力没有进一步变化。这将防止在高应变率下可能导致动荡的情况下，在大厦深处破坏脆性破坏，尽管这可以防止形成裂缝形式的有效除气通道，但在压实状态下不排水的变形将防止孔隙率的降低。渗透性，因此可能有助于将管道横向放气到乡村岩石中。我们通过定义无量纲的达西数来评估排水或不排水变形所需的条件（应变率和渗透率）。封闭的系统或不排水的变形很可能在火山中很普遍（与活动火山系统相邻的岩石中的应变率可能很高，并且质地异质性可能成为流体流动的障碍），因此构成了全面理解力学的重要组成部分建筑物对伴随动荡的应力扰动的反应。 （C）2016 Elsevier B.V.保留所有权利。

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